What causes the wrong value read-out in a Pie menu value slider?
Those numbers are really, unless i'm mistaken, the menu indices. The 7 isn't from the normal_size, but from the index of that pie-item. It's drawing the slider rather awkwardly close to the index. Leading me to believe pie menu's weren't intended to be used for drawing sliders, else it would be a common use case, and have a nice obvious solution. Maybe it does? someone will correct me if there is.

In any case it looks like the pie menus could be more thoroughly documented. I don't think it's a bug rather a misunderstanding of how to use it. It seems mostly geared towards toggling and executing operators.

The only way I know how to suppress the index is by making a row out of the slider.

def draw(self, context):
    layout = self.layout

    mesh = context.active_object.data
    scene = context.scene

    pie = layout.menu_pie()

    row = pie.row()
    row.prop(scene.tool_settings, "normal_size")
    pie.prop(mesh, "show_normal_face")
    pie.prop(mesh, "show_normal_loop")
    pie.prop(mesh, "show_normal_vertex")
    pie.prop(mesh, "show_faces")
    pie.prop(mesh, "show_edges")




All kinds of crazy things can be done with piemenus

Columns

def draw(self, context):
    layout = self.layout

    mesh = context.active_object.data
    scene = context.scene

    pie = layout.menu_pie()      
    col = pie.column()
    col.prop(scene.tool_settings, "normal_size")
    col.prop(mesh, "show_normal_face")




Rows

def draw(self, context):
    layout = self.layout

    mesh = context.active_object.data
    scene = context.scene

    pie = layout.menu_pie()      
    row = pie.row()
    row.prop(scene.tool_settings, "normal_size")
    row = pie.row()        
    row.prop(mesh, "show_normal_face")




box

def draw(self, context):
    layout = self.layout

    mesh = context.active_object.data
    scene = context.scene

    pie = layout.menu_pie()

    # option 1
    box = pie.box()
    col = box.column()
    col.prop(scene.tool_settings, "normal_size")
    col.prop(mesh, "show_normal_face")
    col.prop(mesh, "show_normal_loop")
    col.prop(mesh, "show_normal_vertex")

    # option 2
    box = pie.box()
    col = box.column()
    col.prop(mesh, "show_faces")
    col.prop(mesh, "show_edges")